rs6000: Rework vector float comparison in rs6000_emit_vector_compare - p1

All kinds of vector float comparison operators have been
supported in a rtl comparison pattern as vector.md, we can
just emit an rtx comparison insn with the given comparison
operator in function rs6000_emit_vector_compare instead of
checking and handling the reverse condition cases.

This is part 1, it only handles the operators which are
already emitted with an rtx comparison previously in function
rs6000_emit_vector_compare_inner, they are EQ/GT/GE/ORDERED/
UNORDERED/UNEQ/LTGT.  There is no functionality change.

With this change, rs6000_emit_vector_compare_inner would
only work for vector integer comparison handling, it would
be cleaned up later in vector integer comparison rework.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare_inner): Move
	MODE_VECTOR_FLOAT handlings out.
	(rs6000_emit_vector_compare): Emit rtx comparison for operators EQ/GT/
	GE/UNORDERED/ORDERED/UNEQ/LTGT of MODE_VECTOR_FLOAT directly, and
	adjust one call site of rs6000_emit_vector_compare_inner to
	rs6000_emit_vector_compare.